This invention relates to a permanent magnet pulse generator and particularly to a permanent magnet rotor coupled to the drive shaft of a two stroke engine for generating ignition control pulse signals, and to a simplified method of manufacturing the same.
Various control and operating systems require time spaced pulse signals with respect to the predetermined operational sequence. Ignition systems for internal combustion engines, for example, require energy pulses fed in proper sequence and in time spacing to the spark plugs of the several cylinders in synchronism with the speed of the engine. Recently developed capacitor discharge ignition systems have a capacitor charged from a suitable source and then discharged through a triggered switch such as a silicon controlled rectifier and a pulse transformer to the spark plug. The capacitor may be charged to the desired level through a battery-converter or through an engine driven alternator, the latter being particularly adapted to use in internal combustion engines such as outboard motors, snowmobiles and the like. In either system, a trigger pulse source must be provided for activating of the triggered switch means in appropriate time spaced sequence. Various pulse signal generators which are designed to be driven directly from the engine have been suggested to produce the necessary time spaced trigger signals. An unusually satisfactory functional trigger generator assembly is disclosed in U.S. Pat. No. 3,715,650 which issued on Feb. 6, 1973 to the present inventor. Generally, the generator disclosed therein is applied to a two-cylinder engine and includes an annular permanent magnet rotor unit having diametrically opposite halves of the unit oppositely polarized in a generally axial direction. A pick-up coil is mounted immediately adjacent to the periphery of the magnet unit and in each polarization changeover position a short pulse signal is generated in the pick-up coil unit. In actual practice, as applied to an outboard motor, the rotor unit includes an axially premagnetized ceramic magnet secured to the flywheel hub by a pair of machined clamping members. One clamping member is bonded to and extends from the hub. The annular magnet, with pole shoes secured to the opposite end faces thereof, is assembled on a second machined clamping member which is then threaded into the first clamping member to securely clamp the magnet in position. The clamping members thus project from the lower end of the hub to locate the annular magnet aligned with a pick-up coil unit. This trigger generator provides a very satisfactory pulse source from a functional standpoint and particularly has been found to produce reliable triggering of the ignition system. The various components must be formed with proper machined surfaces and tolerances to provide the desired quality in the assembly commanded by high quality ignition production. This construction is, therefore, somewhat costly.
Further, although the axial polarization of the magnet as disclosed with the flux reversal at appropriate positions on the periphery of the magnet member produces significant pulse signals, the magnets are necessarily preformed and then assembled with the pole shoes to effect the desired axially directed peripheral field. Although a highly functional construction is obtained, the multiple component construction, particularly with requirements of reasonable tolerances, further increase the cost of the rotor unit. The manufacture of the several components, with the required machining to ensure properly abutting faces, as well as the required labor to assemble the several components, thus results in a relatively expensive trigger generator unit. Additionally, the interconnection of the assembly to the lower end of the hub increases the required shaft extension of the engine. Although this does not affect the functioning of the assembly, it does require increasing the profile of the engine power engine unit.
In outboard motors and the like, efforts are generally made to minimize size and, in particular, to lower the profile. The required extension of the rotor unit requires an appropriate increased profile for use of the systems.
On the whole, the trigger generator assembly such as described above is a highly functional assembly or unit which provides a very practical implementation of an alternator-driven, breakerless ignition system. The cost and aesthetic construction, however, have certain adverse characteristics such that it does not provide a truly optimum construction from all practical considerations.